1. Field of the Invention
The present invention relates to an apparatus and a method for fabricating a single-crystal semiconductor by means of Czochralski method, and more specifically, for fabricating a single-crystal semiconductor in which the precipitated oxygen ions and thermal donors are uniformly distributed, thereby reducing the defects in an as-grown single-crystal semiconductor and improving the strength of an oxide film.
2. Description of Related Art
Semiconductor devices are generally formed on a high-purity single-crystal silicon wafer which is, in general, fabricated by Czochralski (CZ) method. The CZ method charges polysilicon blocks in a crucible inside a single-crystal fabricating apparatus and melts the blocks by heaters around the crucible. A seed which is held by a seed holder is then immersed in the melt and lifted with rotating in clockwise or counter-clockwise direction to grow the single-crystal silicon.
The quality characteristics of the grown single-crystal silicon, such as the strength of an oxide film, the amount of oxygen precipitation, and the bulk defects, depend on the heat cycle in the CZ fabrication processes. The heat cycle has therefore been improved in certain related articles. For example, Japanese Laid Open No. 6-211591 and No. 6-211592 disclosed a method for fabricating a single-crystal silicon rod which has uniform precipitated oxygen ions and a firm oxide film. The method utilizes a ring heater whose power is supplied by a heat controller, and which is supported by an electrode, thereby gradually cooling the lifted single-crystal silicon. Moreover, the heat cycle can be modified by adjusting the power supplied to the ring heater.
However, the arrangement of the ring heater causes the following disadvantages.
1. The process cannot immediately response to the degradation of the graphite crucible and the variation lifting conditions such as the position of the melt surface and the condition of introduced gas.
2. In order to prevent the influence to the heater, the crucible must be lowered down when the polysilicon blocks are charged therein. The melt conditions are therefore affected.
3. Since the ring heater screens the seed, the twisting step for preventing stacking faults prior to the growth of the single-crystal cannot be observed.